Such devices have been known for several years and are used, for example, to inject a quantity measurement in internal combustion motors.
The original version of such a device for through-flow measurement was described in DE-AS 1 798 080. This electronically controlled flow meter comprises a main conduit with an inlet and an outlet, in which a rotary positive displacement flow meter in the form of a gear pump is arranged. A bypass runs parallel to the main conduit, via which bypass the rotary positive displacement flow meter can be bypassed and in which a piston serving as a differential pressure sensor is arranged in a measuring chamber. The excursion of the piston in the measuring chamber is measured using an optical sensor to determine the flow rate. The rotational speed of the gear pump is constantly readjusted via an evaluation and control unit based on this signal, the readjustment being such that the piston is always returned to its initial position, if possible, so that only small flows are generated in the bypass. The flow rate within a predefined time interval is calculated in this manner from the number of rotations or partial rotations of the gear pump measured by an encoder and from the known delivery quantity of the gear pump per revolution.
A flow quantity measuring device of this structure is also described in DE 103 31 228 B3. For determining the exact injection quantity profiles, the gear pump is set to a constant rotational speed prior to the start of each injection, so that the movement of the piston is measured subsequently, with this excursion being used to determine the injection profiles. A pressure sensor and a temperature sensor are also arranged in the measuring chamber, the measuring values of which are also supplied to the computing unit to calculate and to correct the injection quantity profiles.
For increasing measuring accuracy, it is in particular necessary, upon the start-up of the apparatus, to free the apparatus from entrapped air which, due to the compressibility of air, leads to significant measuring errors during the measuring process.
WO 2014/11 8041 A1 accordingly describes a flow meter in which a bypass duct is formed at the housing delimiting the measuring chamber of the differential pressure sensor, via which bypass duct a connection is made from the front of the piston to the rear of the piston when the piston abuts against an axial stop at the discharge-side end of the measuring chamber. Entrapped air can thereby get to the discharge-side surface of the piston so that, when the positive displacement flow meter conveys, the air is transported towards the outlet. A check valve is arranged in addition in the bypass duct which prevents a flow from the discharge side to the supply side, i.e., in the opposite direction, so as to be able, after flushing has been performed, to simply move the piston back into its intermediate position upon start-up.
It has been found, however, that existing air entrapments cannot be completely discharged from the flow meter thereby, but that these accumulate in different dead spaces of the apparatus and are only released after some time, which subsequently results in measuring errors. Problems in particular arise from air entrapments in the magnetic clutch with measuring devices that operate at high pressures and whose positive displacement flow meter is driven by an electric motor with an interposed magnetic clutch.